1. Field of the Invention
The present invention relates generally to a liquid crystal display apparatus, and more particularly to an active matrix type liquid crystal display apparatus adapted for use in the precision finders of cameras and projection type televisions.
2. Description of the Background of the Invention
It is known in the art that a matrix type liquid crystal display apparatus is used to utilize the electrooptical effects of liquid crystal to modulate an incident light to picture elements and make up a TV picture and the like. This type of display apparatus includes a liquid crystal layer between two electrodes, that is, a plurality of picture element electrodes arranged in a dot-matrix, and counter electrodes corresponding to the picture element electrodes, wherein the liquid crystal layer optically modulates an incident light in response to an applied voltage.
The matrix type liquid crystal display apparatus, hereinafter referred to as "display apparatus", can take various modes depending upon the kinds of the liquid crystal to be used or the electrooptical properties thereof; for example, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a guest and host (GH) mode, a dynamic scattering (DS) mode, a phase transition mode, and any other suitable mode can be appropriately selected. The display picture elements consisting of the liquid crystal and the picture element electrodes are individually controlled by various methods, for example:
(1) A simple-matrix method; PA0 (2) A method using a non-linear two-terminal device (e.g., a diode) added to each picture element electrode; and PA0 (3) A method using a switching three-terminal device (e.g., thin film transistor (TFT)) added to each picture element electrode.
The methods (2) and (3) are generally called an active matrix system.
The DS mode (Proc IEEE 56 1162 (1968) by G.H. Heilmeier et al), a White Taylor type GH mode (J. Appl. Phys. 45 4718 (1974) by D.L. White et al), and a cholesteric nematic phase transition mode (Proc. SID 13/2 115 (1980) by J.J. Wysocki) are advantageous when they are used in association with an active matrix system using TFT, in that the display brightness is enhanced without the use of a polarizer.
However, this display apparatus has a problem that the addition of a dichroic dye which contains ionizable impurities increases the specific electric conductivity of the liquid crystal layer. As a result, the charge stored in the electrodes between which the liquid crystal layer is sandwiched easily leaks through the liquid crystal layer. To solve this problem, as shown in FIG. 8 a signal voltage storing capacitor C.sub.1 (i.e., an additional capacitor) is provided in parallel with a picture element capacitor C.sub.2 connected to a drain electrode of the TFT, so as to increase the capacity of the capacitor C.sub.1. In this way the electric charge is maintained.
However, the method using the signal voltage storing capacitor C.sub.1 basically has a limitation in maintaining the charge, and in order to drive a highly integrated matrix display apparatus having many capacitors C.sub.1 of sufficient capacitance, the source driver and the switching TFT must supply a very large current, and the resistance of the source bus line must be very low. In addition, the space for accommodating picture element electrodes is accordingly reduced, and manufacturing difficulty is involved.
FIG. 9 shows an example which drives an equivalent circuit in a similar manner to the present invention. That is, an active matrix type liquid crystal display apparatus. The apparatus is provided with picture elements each of which comprises two capacitors C.sub.2 and a three-terminal element such as TFT, wherein the picture elements and the TFT are arranged in a matrix ("Japan Display Digest", page 80 to 83).
In this display apparatus the pairs of picture element electrodes are connected to the source and drain of each TFT and counter electrodes T.sub.R and TD corresponding to the respective picture element electrodes. The electrodes T.sub.R are connected to a reference electrode line R, and the electrode T.sub.D is connected ed to the data electrode line D. The reference line R is earthed or maintained at a constant level of voltage, and the data electrode line D is applied with a signal voltage in accordance with the information to be displayed. Under this arrangement, when the gate voltage is at a high level, the TFT is on, thereby forming a closed circuit starting from the data electrode line D, the liquid crystal layer, the first picture element electrode, the TFT, the second picture element electrode, the liquid crystal layer, and ending at the reference electrode line R. As long as the gate voltage is at a high level, the capacitors C.sub.2 and C'.sub.2 are charged. However, this charge does not last, and gradually discharge through the liquid crystal layer and switching three-terminal element when the TFT is off. In order to maintain the charge in the capacitors sufficient to drive the liquid crystal properly, it is required to prepare liquid crystal having an extremely high specific resistance.
To solve the problem pointed out above, the inventors have made an invention shown in FIG. 7, for which Japanese Patent Application No. 1-95581 is pending laid open on Jul. 11, 1990 as No. 2-272521. According to the prior application, the signal volt storing capacitor C.sub.1 is separated from the picture element capacitor C.sub.2 by the TFT.sub.2. The TFT.sub.1 and the capacitor C.sub.1 function as a sampling/holding circuit. While the TFT.sub.1 is on, the capacitor C.sub.1 is charged with the signal voltage through the drain of the TFT.sub.1, and after the TFT.sub.1 is off, the capacitor C.sub.1 keeps the signal voltage being applied to the gate of the TFT.sub.2 until the next signal voltage is sampled through the drain. The advantage of this method of FIG. 7 is that even if the liquid crystal has a small specific resistance, the display is protected against an unfavorable influence of discharge in the liquid crystal. However, when the halftone image is to be displayed, a problem arises. In FIG. 7, suppose that the capacitor C.sub.2 is not charged, that is, the picture element "a" and the counter electrode "b" are at an equal potential, and that the capacitor C.sub.1 is negatively charged. In this situation, the gate potential of the TFT.sub.2 is kept negative, the TFT.sub.2 is off, and the picture element electrode "a" is electrically isolated from the earth line. At this stage, an a.c. voltage V.sub.C is applied to the counter electrodes, the potential of the picture element electrode a changes accordingly.
Then, when the capacitor C.sub.1 is positively charged, the TFT.sub.2 is on. The capacitor C.sub.2 is charged at a time constant .tau..sub.ON =C.sub.2 R.sub.ON. At this stage, the potential of the picture element electrode a has the same polarity as V.sub.C, and gradually becomes equal to the earth potential. When the halftone image is to be displayed, it is required to repeat the polar reversion of the voltage V.sub.C in a shorter period of time than the period of time required for the completely charging of the capacitor C.sub.2.
However, the polar reversion of voltage V.sub.C changes the potential of the drain, thereby changing the "on" resistance R.sub.ON of TFT.sub.2. Even if the positive/negative symmetrical a.c. voltage is applied as the voltage V.sub.C, the operating point of the TFT.sub.2 unavoidably changes with time, thereby causing the "on" resistance R.sub.ON to have different values between the positive half-cycle and the negative half-cycle. Thus, the potentials of the picture element electrode "a" and the counter electrode "b" are non-symmetrical for negative and positive, thereby bringing a direct current component into the voltage applied to the liquid crystal layer of the capacitor C.sub.2. This d.c. component causes a flickering in the pictures on the screen.
When the liquid crystal layer has a d.c. component, the liquid crystal and electrodes are liable to electrolysis. This is detrimental to the formation of clear image.